IONIZATION AND TEMPERATURE OF NOVA SHELLS .2. THE INFLUENCE OF REALISTIC PHOTOSPHERIC RADIATION-FIELDS

In order to determine the physical situation preceding the process of dust formation in classical novae we calculate the time dependent ioni zation and temperature structure of an expanding shell which is expose d to the ionizing radiation of a hot central source. Additionally the energy balance between ionizational heating and cooling processes by e lectron recombination, gas expansion, collisionally excited forbidden lines of several atoms and free-free radiation of hydrogen is solved. The radiation field of the classical nova is given by a synthetic spec trum calculated on the assumption of spherical symmetry and a time-ind ependent, expanding, non-LTE atmosphere in radiative equilibrium We sh ow that in novae of slow and moderate speed class complete carbon ioni zation throughout the nova shell is not possible before day similar to 50 after the outburst. In the early phases the degree of carbon ioniz ation is significantly lower than in the corresponding blackbody model , thus enabling a complex chemistry to proceed, which may involve fast ion-neutral reactions.